1. Field of the Invention
The invention relates to a mixture control for an internal combustion engine with a lambda probe and with a memory device containing nominal values for the fuel supply dependent on operational values of the engine, such as load, rotational speed and temperature.
2. Description of the Prior Art
For the mixture control, it is known to rely the so-called lean burn concept which involves the use of an air surplus and leads to an optimizing of the fuel consumption. As is known the NO.sub.x values are reduced in this operating mode. It is possible that the HC values are slightly increased. For such operation in the lean burn range, it is necessary to provide a lambda probe in the exhaust gas duct. The lambda probe is adapted to measure directly the oxygen content in the exhaust gas or to determine the lambda value in another manner.
DE-OS No. 33 21 424 describes a mixture control wherein, on the one hand, the nominal value of lambda=1, and, on the other hand, a nominal value of lambda for the lean burn mode may be given. These two nominal values are processed in separate control loops. A shifting between these two nominal values is effected parameter-dependent. On the basis of such an operation with two lambda nominal values a sufficient optimizing cannot be reached. Particularly in the lean burn range the presentation of only one nominal value of lambda for all the operating conditions is insufficient in order to guarantee the driveability.
In addition, thorough investigations and checks have disclosed that by setting a fuel injection value according to the nominal value of lambda an optimum moving behavior cannot be reached due to the fact that the differences between various specimens of the internal combustion engines are too pronounced. A compensation of the spread between such specimens is not possible on the basis of that concept.
A mixture control is also known from DE-OS 32 48 745. Therein basic values of lambda are corrected according to the ascertained fluctuations of rpm. These corrected values serve as desired values for a control. The corrections do not always provide unequivocal operating values so that the engine does not always operate in an optimum manner.
US-PS No. 45 66 419 describes a control for the air/fuel ratio of an internal combustion engine. A ROM stores nominal values for the injection time of the injection valve and also nominal values of lambda. A microprocessor calculates correction values in relation to operating parameters and to the actual value of lambda. Then a corrected injection time is calculated. This method requires a number of calculating steps so that it is difficult to calculate the corrected values within each revolution, especially for higher speeds.